1. Field of the Invention
This invention relates to a hydrophilic polymer, a method for the production thereof, and uses found therefor. More particularly, this invention relates to a hydrophilic polymer assuming hygroscopicity owing to the inclusion in its molecular unit of a carboxyl group, a carboxylate group, and a hydroxyl group and nevertheless having its own water absorption ratio being 1 to 30 times to the own weight owing to the mutual reaction of the carboxyl group and hydroxyl group, a method for the production thereof, and uses found therefor.
2. Description of the Prior Art
In recent years, the so-called water-absorbing polymer, i.e. such polymers which are insoluble in water and capable of absorbing water, have come to be used widely in the field of civil engineering, the field of agriculture and forestry, the field of sanitary articles, etc. which by nature are allowed to take full advantage of the polymers' ability to absorb and retain water.
For the production of these water-absorbing polymers, a method which simultaneously polymerizes and self-crosslinks a hydrophilic monomer in the presence of a crosslinking agent (Japanese Patent Publication SHO 54(1979)-30,710), a method which polymerizes a hydrophilic monomer in the presence of a small amount of a crosslinking monomer (Japanese Patent Laid-Open SHO 58(1983)-71,907), a method which polymerizes partially neutralized acrylic acid in the presence of a water-soluble polyhydric alcohol and a surfactant (Japanese Patent Laid-Open SHO 55(1980)-108,407), a method which causes a polyepoxy compound to react with a copolymer comprising an alpha-olefin and/or a vinyl compound and maleic anhydride (Japanese Patent Laid-Open SHO 56(1981)-36,504), and a method which polymerizes a monomer containing 2-hydroxyethyl acrylate and acrylic acid or a metal salt thereof in the presence of a crosslinking agent with the monomer concentration kept over 35% by weight and, during the course of the polymerization, causes self-crosslinking of the monomer in process of polymerization (Japanese Patent Laid-Open SHO 56(1981)-161,413), for example, have been proposed.
The water-absorbing polymers obtained by these known methods have water absorption ratios so large as to take up and hold purified water approximately to 200 to 1,000 times their own volume and, therefore, are uses for which these polymers prove to be totally unfit because their characteristically large water absorption ratios are rather detrimental than otherwise. When these water-absorbing polymers are used in sheets for preventing dew condensation, sheets for regulating moisture, or fabrics for absorbing perspiration and moisture as attached fast to their fibers by a suitable means, they are often swelled to an unduly large extent and take a long time before they release the absorbed water by evaporation and resume their former dry state. At time, they are overswelled possibly to a point where sheets sustain fracture or the polymers come off the sheets or fibers. This phenomenon is particularly conspicuous when the water-absorbing polymers are amply used for the enhancement of hygroscopicity and ability to prevent dew condensation.
When water-absorbing polymers of high absorption ratios are used in water stop packings which comprise rubber and a water-absorbing polymer incorporated in the rubber, the packings absorb water possibly in such a large volume as to swell their rubber matrixes excessively and require a very long time before they are fully dried. In an extreme case, the abosorbed water may cause fracture of the rubber matrixes and deprive the packings of usefulness.
Water-absorbing polymers are generally handled in the form of powder and, therefore, are prone to entail a disadvantage that the powder is caked with absorbed moisture or drifted readily in the form of dust. This disadvantage can be eliminated in the case of water-absorbing polymers which are capable of being handled in the form of aqueous dispersion or aqueous slurry. Unfortunately, however, the conventional water-absorbing polymers of high water absorption ratios are not easily transformed into an aqueous dispersion or aqueous slurry. In the circumstances, a hygroscopic polymer which has the water absorption ratio thereof lowered to a point where the polymer used in a water stop sheet, for example, does not suffer the moisture or water absorbed therein to cause either fracture of the matrix of sheet or component fibers thereof or separation of the polymer from the sheet and which is capable of being transformed into an aqueous dispersion has a prospect of finding a great demand.
A water-absorbing polymer having the capacity for water absorption repressed ideally as described above could be obtained from the conventional water-absorbing polymer by suitably increasing the crosslinking degree thereof. When this increase of the crosslinking degree is effected by a method using a polyhydric alcohol in a large amount, the treatment must be performed at an elevated temperature for a long time for the polymerization degree to be sufficiently heightened. Further, the polyhydric alcohol suffers from low crosslinking efficiency because it readily vaporizes or passes into the ambient air. When the increase of the crosslinking degree is performed by a method using in a large amount a crosslining monomer possessing two or more unsaturated groups in the molecular unit thereof, the crosslinking proceeds unevenly and the crosslinking monomer which is noxious inherently remains in its unaltered form because the crosslinking monomer is sparingly soluble in a water-soluble (hydrophilic) monomer. Thus, this method suffers from poor safety and high cost. In the case of a method which effects the desired increase of the crosslinking degree by the reaction of a polyepoxy compound with a water-soluble polymer, there arises a possibility of the polyepoxy compound remaining after the reaction and inducing the drawbacks of toxicity and high cost.
As builders for detergents, phosphates, water-soluble high molecular electrolytes, and zeolites have been known heretofore to the art. Of these detergent builders, phosphates and water-soluble high molecular electrolytes are soluble in water and excellent in ability to sequestrate polyvalent metal ions in hard water, ability to purge fabrics of solid particulate stain, and ability to prevent removed defiling particles from adhering again to fabrics in process of cleaning. From the standpoint of preserving water from pollution, the phosphates have disadvantage that they possibly form a cause for the phenomenon of eutrophication of water. The water-soluble high molecular electrolytes which may be represented by sodium salts of carboxylate such as polyacrylic acid, polymaleic acid, and acrylic acid-maleic acid copolymer are deficient in biodegradability and, to be sufficiently effective, must be used in a large amount and inevitably are disadvantageous in that they entail environmental pollution. In contrast, the zeolites which are a water-insoluble inorganic particulate substance do not cause so heavy water pollution as the aforementioned phosphates and water-soluble high molecular electrolytes but are deficient in ability to sequestrate polyvalent metal ions. Recently, the problems such as undue abrasion of washing machines by zeolites have come to arouse serious concern.
In the circumstances, several water-insoluble organic detergent builders improved in ability to sequestrate polyvalent metal ions have been proposed. For example, West German Patent No. 2,055,423 discloses a method which uses a polymer crosslinked as with divinylbenzene, West German Patent No. 2,216,467 discloses a method which uses a cation exchanger obtained by impregnating a flat fibrous matrix as with a polymer containing a carboxyl group, and West German Patent No. 2,307,923 discloses a method which uses what is obtained by impregnating a porous matrix with a mixed monomer of a crosslinking agent containing two or more double bonds and an ethyleic double bond and subsequently polymerizing the mixed monomer lodged in the matrix. The polymers involved in these proposed methods are invariably water-impregnable crosslinked polymers obtained by using as a comonomer a crosslinked monomer containing two or more double bonds.
These water-impregnable crosslinked polymers, however, have a disadvantage that they exhibit a weak gel strength in an impregnated state and, during the course of laundering, undergo disintegration and induce unwanted adhesion to laungered articles when the crosslinking monomer is used in an unduly small amount. If the amount of the crosslinking monomer to be used is unduly large, they have a disadvantage that for a certain unknown cause, the crosslinking monomer survives the reaction possibly to a point where the safety of polymer is jeopardized. Further, since not so large amount of the crosslinking monomer is soluble in water, a method which comprises conducting the polymerization in the state of an aqueous solution of unneutralized carboxylic group-containing monomer having the crosslinkable monomer dissolved therein, and after the completion of the polymerization, neutralizing the resultant polymer has been proposed. This method, however, inevitably grows heavily in intricacy. The crosslinking monomer itself is very expensive. Thus, these polymers have not been fully developed to a practicable level.
Moreover, such water-soluble organic high molecular compounds as agar, gelatin, polyvinyl alcohol, polyethylene glycol, and partially crosslinked derivatives thereof which have been heretofore used as a coolant have a nature such that when they are cooled below 0.degree. C. in their water-containing state, the resultant gels are frozen hard and deprived totally of flexibility. Thus, they have a disadvantage that when they are used in cooling human bodies and foodstuffs, they exhibit a poor ability to come into close contact with the contours of the objects being cooled. During their use on human bodies, the frozen gels impart unpleasant sensation due to the hardness. When the frozen gels are used on foodstuffs, they inflict injuries to the foodstuffs or their wrappers. A proposal has been made to use as a coolant a hydrated gel of a highly absorbent resin such as slightly crosslinked sodium polyacrylate which takes up water approximately to 100 to 1,000 times its own weight. The hydrated gel has found favorable utility in products such as paper diaper, sanitary napkin, and agricultural-horticultural coolants which require a large capacity for water absorption. For the function as a coolant, the hydrated gel's large ability to absorb water is not utilized to advantage. Conversely, the hydrated gel is not fully satisfactory in the flexibility in a frozen state which is an important requirement for a coolant. Regarding the function to be expected of a coolant, the capacity for absorption of water is not required to be very large. The hydrated gel, however, is not fully satisfactory in flexibioity, an important requirement to be satisfied by the hydrated gel destined to be used in a frozen state as a coolant.
For the elimination of these drawbacks of the conventional products as described above, there has been proposed a method which produces a gel incapable of freezing by causing the aforementioned water-soluble high molecular compound or highly absorbent resin to absorb an aqueous solution containing a polyhydric alcohol. This gel is deficient in the quality of a coolant because it is incapable of utilizing the latent heat of melting of ice. The coolants using the conventional highly absorbent resins have a disadvantage that when they are left standing under the sunlight or at elevated temperature for a long time, they gain much in viscidity and lose softness in a great measure during the course of freezing.
As a coating material for proofing a given substrate against dew condensation, there has been proposed a product which is vested with a water-absorbing property by the addition of an absorbent body pigment such as diatomaceous earth, pearlite, or zeolite beside such ordinary paint components as synthetic resin emulsion, coloring pigment, pigment dispersant, tackifier, fungicide, and antiseptic (Japanese Patent Laid-Open SHO 57(1982)-151,661). This coating material is deficient in ability to prevent dew condensation because it has no satisfactory ability to absorb water. If the absorbent body pigment is used in a large amount for the purpose of enhancing the ability to absorb water, the coating material is no longer capable of forming a film rich in strength.
For the elimination of the drawbacks suffered as described above by the coating material capable of proofing a substrate against dew condensation owing to the use of an absorbent body pigment, there has been proposed a coating material adapted to proof a substrate against dew condensation by the incorporation of a highly absorbent resin (Japanese Patent Laid-Open SHO 62(1987)-205,171 and SHO 62(1987)-265,364). This coating material for proofing substrates against dew condensation owing to the incorporation of a highly absorbent resin enjoys an improved ability to absorb water and consequently exhibits an improved ability to preclude dew condensation. The highly absorbent resin has a capacity for taking up water approximately to 50 to 1,000 times its own weight. Since it absorbs water excessively, it requires a long time in releasing the absorbed moisture. Since it is swelled greatly with absorbed water, the film formed with the coating material is deprived of surface smoothness by addition of only a small amount of water. The film sustains cracks because the difference of swelling and contraction of the film during the repeated cycles of admission and release of water is large. In the case of a water paint, if this paint incorporates therein the highly absorbent resin in a required amount, it acquires unduly high viscosity or undergoes heavy gelation to a point where the produced paint is effectively applied to a given surface only with difficulty.
An object of this invention, therefore, is to provide a novel hydrophilic polymer, a method for the production thereof, and uses for the polymer.
Another object of this invention is to provide a hydrophilic polymer having 1 to 30 times of absorption ratio to the own weight.
Yet another object of this invention is to provide a novel cation sequestrating resin, a coolant, and a coating material for proofing a substrate against dew condensation.